Thermal efficiency has always been an important consideration in the design and operation of water heaters. This is even more so the case today when the cost of energy continues to climb and its availability can no longer be assumed to be without limit.
The efficient use of the energy source to heat the water has two distinct aspects. The first is the efficient transfer of the heat energy from the heat source to the water being heated; the second is the maintenance of the water inside the water heater at its heated temperature with as little heat as possible being transferred from the water heater and the water contained therein to the surrounding environment.
The present invention is concerned primarily with the second aspect, that is to say, the minimization of heat losses from the water heater to the surrounding environment.
As is well known, the rate of heat transfer from a water heater to the surrounding environment, whether it be by convention, conduction or radiation, is always dependent upon a temperature difference.
In the case of a conventional electric water heater, the heat transfer loss (otherwise known as "standby power consumption") is a linear function of the difference between the mean ambient air temperature and the mean heated water temperature, with the coefficient being related only to the thermal integrity of the outer shell of the heater. Thus, in the case of an electric water heater, there are several ways to reduce the heat loss: (1) increase the shell insulation surrounding the water heater water tank, (2) install the water heater in a relatively warm location, and (3) operate the water heater at a water temperature that is as cool as possible (although in that regard, it should be noted that the average household requires hot water of at least 140.degree. F. for the proper operation of automatic dishwashers and other water consuming appliances).
In the case of conventional gas water heaters, similar factors apply; however, a gas water heater utilizes the ambient surrounding air for the combustion of the gas and the resultant exhaust gases must be vented through a flue, causing an additional loss of heat and energy. These flue losses are dominated by natural convection which varies approximately with the 1.3 power of the difference between the mean ambient air temperature and the mean heated water temperature. By employing an automatic flue or vent damper (which restricts the flue passageway inside the water heater tank or the vent passageway located above the heater--at least while the water heater is in a standby mode with the burner off and only the pilot in operation), the flue losses in a gas water heater can be somewhat minimized; however, it can be said that a gas heater will be more sensitive to the difference between the mean water temperature inside the heater and the temperature of the surrounding ambient air than is ture for an electric water heater (or other unvented water heater not utilizing combustion to heat the water).
In recognition of the foregoing basic physical principles, modern energy saving water heaters are provided with better insulation than their predecessors and consumers are encouraged to operate the heaters at a lower water temperature and to pay greater care to the heater's location and installation. Additionally, it is now possible to purchase insulation blankets for installation on older water heaters and to purchase damper kits for installation of automatic flue or vent dampers on gas water heaters.
Furthermore, it has been proposed to provide the water heater with an automatic control system to lower the operating temperature to which the hot water is heated during periods of low hot water demand and to raise the temperature back up during periods of high demand in response to signals from either a single temperature sensor connected to the hot water outlet pipe or from both such an outlet sensor and a second temperature sensor on the cold water inlet pipe, with the latter sensor acting as a veto switch for preventing the heater from erroneously switching into a high demand mode when a low demand situation has actually remained in effect for several hours, as taught by Scott U.S. Pat. Nos. 4,016,402 and 4,166,944, respectively.
The theory behind employing two different operating temperatures in the same water heater installation, a lower standby (or low demand) temperature and a higher operational (high demand) temperature, is that in the typical residential or commercial application, the hot water heater is called upon to supply hot water in quantity only during certain times of the day--for instance, in the morning when the family is waking up and undergoing their morning toilet; in the mid-morning when the automatic clothes washer is in operation, and in the early evening when the dishwasher has been put into operation following the evening meal. During such periods of high demand, relatively hot water is required to be output by the hot water heater; first of all, to provide the high sanitizing temperatures required by automatic washing machines and dishwashers; secondly, to increase the effective capacity of the heater by permitting a greater amount of cold water to be mixed with the hot water heater's output when lukewarm rather than hot water is all that is required.
However, the nature of the particular water heater being used, the location which it is installed in, whether the plumbing is of copper or iron, the climatic conditions, the season of the year, whether the hot water supply system is of a forced recirculating type, of a convection recirculating type, or a nonrecirculating type, the temperature of the cold water as supplied by the water company, and other similar considerations all obviously affect the temperature to which the temperature sensor on the output pipe should be set (and, if employed, the temperature on the cold water inlet pipe), in order to optimize the energy saving and to minimize any inconvenience to the consumer.